AS EVERYONE who has ever felt fat knows, we are suckers for any new theory that promises weight loss. The people who dream up the diet theories realize this, a fact to which recent diet books bear abundant witness. In the current crop, everybody's got a foolproof gimmick, from the "appestat" and "setpoint" to "positive neurotransmitters" and "mood foods," from "fruit fantasies" to "protein powder" to "Slimming Daily Allowances." Some diets seem so patently absurd that it matters not that you can actually lose weight on them; it's impossible to stick to them. Just imagine eight weeks of eating two "protein shakes" and one "main meal" per day as prescribed by Richard A. Passwater in the Slendernow Diet (St. Martins, $11.95). The shakes, which Passwater says "insure a balanced, nutritious meal of controlled measured amounts of food" (remember Metrecal, dieters?) are made of commercial protein powder, fruit juice or milk, oil, ice, sugar substitute and an optional raw egg plus any of some 101 combinations of flavoring agents -- dieter's choice -- from chocolate and brandy extracts with cinnamon, to carrot juice, to grape juice with 7-Up and bran.

The more outrageous the gimmick, it seems, the better the book sells. Judy Mazel and her Beverly Hills Diet -- in which it's not what you eat but when and in what order -- are thriving proof. By prescribing weeks of enough fruit to make an ape long to evolve, Mazel won so many fans that she has come out with a sequel, an incredibly complex eating formula called the Beverly Hills Diet Lifetime Plan (written with Susan Shultz, Macmillan, $13.95). In it, she promises readers that by adhering to her maze of charts, lists, rules, "antidotes," "precidotes" and "corrective countermeasures" they can pig out on hot fudge, malteds, anything, and as long as they eat only watermelon all the next day, they will stay thin. The book, including its recipes (by Nancy H. Marcantonio, a caterer-convert), is based on Mazel's "conscious combining" principle: fats can be eaten with anything; carbohydrates can't be eaten with proteins. In other words, load the eggs benedict with hollandaise, but don't let the english muffin pass your lips and never, never dredge your veal scallops with flour before you saut,e. With such license, Beverly Hills gives us recipes for everything from bearnaise to pizza to a butterflied leg of lamb in a mustard-herb marinade (that is uncannily faithful to a popular one by Julia Childs).

While Mazel's bizarre manipulation of physiology and chemistry may not be scientific, it is trendy. And bestselling diet books are nothing if not trendy. These days, the fashion is to couch the book's come-on, the gimmick, in scientific--preferably biochemical -- terms, actually buzzwords that the reader thinks he's "been hearing a lot about lately." The Southampton Diet by Stuart Berger (with Marcia Cohen, Simon & Schuster, $15.50) typifies this style. Berger bases his diet and recipes on a "biochemical link" between mood and food. There are, Berger explains, 22 amino acids that bring the brain positive or negative information; these amino acids are digested in certain foods and are carried to the brain by means of enzymes contained in certain other foods. Thus, there are "happy foods" (that "contribute to your happiness and self-esteem and truly satisfy your hunger") and "sad foods" (that "may tend to depress you and actually make you hungrier"). "Happy foods," we notice, are merely the turkey, chicken (skin removed), cottage cheese and whole grains that eternally stare up from a dieter's plate, while "sad foods" are the likes of sugar, mayonnaise, marbled meats and chocolate. To confuse -- or encourage -- the seasoned dieter, Berger also labels lobster, pickled herring, chianti and aged beef as "sad." Lobster depressing?

Also right in style now are the diets that emphasize nutritional balance and vitamin intake. But surprisingly, some books based on these themes tend to overdo it. In her Vitamin Diet for Quick and Easy Weight Loss, Francine Prince (Cornerstone/Simon and Schuster, paperback, $7.95) teaches how to "reset" the "appestat." Theoretically located in the hypothalamus at the base of the brain, it tells us when we are hungry and when we are full. The "quick" weight loss comes with a two-week high-protein, low-carbohydrate and low-fat diet supplemented by vitamin pills (a "Slimming Daily Allowance", or SDA). As Prince admits, the diet drains the body of all excess water causing a loss of up to 12 pounds a week, and in the process, valuable vitamins and minerals are washed away -- thus the supplement of pills. This is a classic "crash diet" -- the kind that fads are made of, that make celebrities of their inevitably once-fat creators and keep America buying diet books.

After Prince's initial crash comes "maintenance," in which no portion sizes are specified but because your "appestat" has been reset (is this what we used to call "shrinking the stomach"?) you are sensitive to your "full" point. Prince says her recipes are "low-calorie, low-fat, low-saturated-fat, low-cholesterol, high-fiber cooking and baking . . . using no sugar or salt!" That doesn't seem to leave much, yet Prince's recipes are excruciatingly long: one saut,eed chicken recipe calls for 15 ingredients, including a chicken stock made from another 12 items.

Vaguely similar to the appestat theory is that of the "setpoint" offered by William Bennett and Joel Gurin in The Dieter's Dilemma (Basic, $14.95). A gloomy hodgepodge of theories unconnected to any specific diet, Dilemma offers a "setpoint" for body fat determined by a built-in control system that works like a thermostat. About 200 pages are devoted to explaining how increased activity lowers the setpoint. Another section debunks many other theories about weight loss, destroys the credibility of most diets and "diet doctors" and sarcastically describes "how to write a diet book." Dilemma extolls exercise, but finds reasons why in some cases even that sacred cow of weight loss doesn't work.

Not so Richard Simmons, now the darling of a.m. TV, who is cashing in on the success of his first book with the Never-Say-Diet Cookbook (Warner, $15.95). More innovative in style than substance, Simmons tosses out flip patter that is corny and very silly, but he does force the dieter to laugh at herself (he clearly speaks to women). But underneath his flamboyance Simmons' "Live It" (as he calls his die-et -- get it?) presents a no-nonsense approach to weight loss: eat anything but control the portions. And exercise. His recipes are as sensible as his advice.

More wordy and less precious than Simmons, but at least as firmly rooted in common sense, is Martin Katahn's The 200 Calorie Solution, (Norton, $13.95). No crash diet here; Katahn simply eliminates unnecessary fatty foods and sugar and increases fruits, vegetables and whole grains while "adding an average of 200 calories of energy expenditure to your life each day." However, his book is not for people accustomed to working out for three hours a day with Jane Fonda. The exercises he prescibes seem to be for overweights who have been habitually sedentary except when they walk to the corner for a Big Mac and fries.

Two new, useful and entertaining books for weight watchers -- even those not on a specific diet -- are How to Eat Like a Thin Person, by Lorraine Dusky and J.J. Leedy, (Simon and Schuster, $13.50; paperback, $5.15) and Successful Dieting Tips, by Bruce Lansky (Meadowbrook, paperback $4.95). Approaching their subject with humor, both books pull together scattered suggestions and information experienced dieters will recognize. Lansky writes in short, punchy lists and single paragraphs reminding us not to "keep old large- sized clothes 'just in case.' You're committing yourself to failure if you do"; and to buy whipped butter or margarine -- a little bit spreads a longer way. Dusky and Leedy include a chapter called "Smoking Versus Weight or I'll Get Fat If I Can't Have a Drag," in which they say new non-smokers' blood supply increases, adding up to two pounds of fluid, and suggest that quitters drink lots of water, brush after each meal, breathe deeply and eat fruit to combat the gain.

At the "food and nutrition" end of the current diet book spectrum is the ponderously longwinded The Sugar Trap, by Beatrice Trum Hunter (Houghton Mifflin, $10.95). Hunter's thesis is that we consume a dangerously high level of sugar, sometimes without knowing it. She discusses in tedious detail each of the natural and artificial sweeteners -- their composition, processing and nutritional value or lack of it. Hardly a book to laze on the beach with, it is a useful reference. One can imagine citing it when confronting health food fanatics or righteous consumer advocates who press their favorite products upon us. For instance, Hunter reminds us that raw sugar as produced in this country "actually is white sugar with traces of cane or beet pulp added back to give it approximately the same appearance and taste as genuine raw sugar (which the FDA has outlawed as being too contaminated for safe consumption). Hunter includes one fascinating section on possible natural and artificial alternative sweeteners for the future such as Katemfi, sometimes called "the miraculous fruit of Sudan," and a sweetener composed of left-rotating molecules that are not digestible by humans (whose enzymes, says Hunter, can metabolize only right-rotating molecules). After all this, Hunter belabors the obvious in concluding that we do not need more alternative sweeteners, but need to consume less of all sweeteners and all foods (like soft drinks) that require high levels of sweeteners.